Material processing technologies using plasmas generated in the discharge of reactive gases, such as etching and CVD(chemical vapor deposition), are being widely employed in industries and have taken root as important basic technologies. For further advancement of these technologies, it is strongly desired to make precise measurement of plasma states, especially plasma electron densities, as their basic information, and make a definite grasp of their sizes, spatial distributions and changes with time for proper control of plasma states. However, it cannot be said that the technique for measuring plasma electron densities has been well established to satisfactorily meet the needs from industries.
A classical method for measuring plasma electron densities employs a “Langmuir probe”, as shown in FIG. 17. In this method, a metal electrode 82 is inserted in a plasma 81 generated in a plasma vessel 80 and the current is measured, which is generated when a direct current voltage is applied to the electrode 82. This method is very effective as well as convenient for discharge plasmas of argon, hydrogen, nitrogen and the like which yield no film deposition. In practical material processes using reactive plasmas, however, the surface of the metal electrode 82 inserted in the plasma 81 is covered with a deposition film, which often causes deterioration of the voltage-current characteristics. Therefore, it is difficult to employ a Langmuir probe in material processes using reactive plasmas. In addition, since heavy metal contaminants are emitted from the Langmuir probe, it is particularly difficult to apply the probe to semiconductor processes.
As a method which is unaffected by metal contamination and thin film deposition, the “microwave interference method” has been known, in which microwaves are irradiated from an incident antenna 83 to a plasma 81 and the microwaves transmitted through the plasma 81 are received at a receiving antenna 84, as shown in FIG. 18. The plasma electron densities are obtained from measurement of the phase difference caused by transmission of microwaves through the plasma 81. However, this technique has the following demerits. The method requires large windows for incidence and transmittance of microwaves as well as a large size of plasma 81 and it can only obtain the mean density of electrons along the passage of microwaves(spatial resolution is unobtainable). In addition, the measuring apparatus is expensive.
On the other hand, a highly sensitive method for measuring plasma electron densities by using a “surface wave probe” (also called plasma absorption probe) has been recently developed, which is unaffected by thin film deposition, yields no emission of metal contaminants and provides a sufficient spatial resolution (see, for example, Patent Document 1, Japanese Unexamined Patent Publication(KOKAI) No. 2000-100599).
In this method, surface waves propagating along the surface of a rod type surface wave probe 85 inserted in a plasma 81 are excited by microwave signals transmitted from a network analyzer 86, as shown in FIG. 19. The surface wave probe 85 houses a coaxial cable and a loop antenna connected with the cable in a dielectric tube. At a specific frequency f0, decided by the electron density, the surface waves become resonant standing waves and are strongly excited. At this instant, the signals reflecting from the surface wave probe 85 decrease their intensities resonantly and can be observed by the network analyzer 86. Thus, the electron density can be obtained from measurement of the resonant frequency f0.
The method using this surface wave probe can be widely applied to reactive plasmas. It is applicable to electron densities from 108 cm−3 to 1012 cm−3 and discharge pressures from 10−5 Torr to 10 Torr.
The spatial distribution of electron densities can be measured with a resolution of several mm, by moving a surface wave prove 85 which is inserted in a plasma 81 through a port hole of a vessel 80. This function provides an important means for research and development in which a detailed survey is required for search of the optimum conditions.
However, in volume production under fixed conditions, the need is low to measure the spatial distributions of electron densities with high resolutions as minute as several mm. Reversely, when a foreign body such as a surface wave probe is protruded into a plasma in a volume production equipment, the plasma is likely to be disturbed during the plasma process. And, when the plasma vessel is cleaned after the process, the surface wave probe 85 protruding into the vessel is likely to be damaged.
To cope with the difficulties, a plan may be thought, in which the conventional rod type surface wave probe is retracted to a position where the tip of the probe is flush with the wall surface of a plasma vessel. However, when the probe is retracted to the vicinity of the wall surface where the electron density is small, significant signals are hidden by noises, leading to inaccurate measurement.
On the other hand, another method to measure plasma electron densities has been known, which employs a metallic dipole antenna and utilizes the resonance of electromagnetic waves (see Non-Patent Document 1: R. L. Stenzel, Rev. Sci. Instrum. 47, 604 (1976) and Non-Patent Document 2: R. B. Piejak, V. A. Godyak, R. Gamer, B. M. Alexandrovich and N. Stemberg, J. Appl. Phys. 95, 3785 (2004)).
In general, the wave length λ of electromagnetic waves, not limited to plasmas, propagating through a medium space with a dielectric constant ∈ is given by λ=c/(∈½), where c is the light velocity in vacuum. Consider a T-shape antenna, in which a metallic wire with a length L is connected to the core conductor of a coaxial cable and another metallic wire with the same length L is connected to the surface conductor of the cable. When the antenna is placed in a medium space with a dielectric constant ∈ and an electric power with a frequency f is sent to the antenna, electromagnetic waves are resonated at a frequency when L=λ/4 and the electric power is stored in the antenna. This kind of antenna is called a dipole antenna. For given dipole length 2L and dielectric constant ∈, the resonant frequency is given byfr=c/(4L∈½)   (1)In the simplest approximation(cold plasma model with no collisions) for a plasma space, the dielectric constant of the plasma is given by the following equation.∈=1−(fp2/f2)   (2)
Here, fp is a physical quantity called electron plasma frequency and is given by the following equation.fp=(½π)·(e2ne/me∈0)½  (3)where e and me are the electrical charge and mass of an electron, respectively, ∈0 is the dielectric constant of vacuum and ne is the electron density.
The resonant frequency fr of a dipole antenna in a plasma can be determined by substituting equations (2) and (3) into equation (1). If f0 denotes the resonant frequency in vacuum, free from plasmas, the following relation is obtained.fr2=f02+fp2  (4)
Therefore, the electron density ne can be determined from the difference between two measured data of f0 (GHz) and fr (GHz), as expressed by the following equation.ne={(fr2−f02)/0.81} (1010 cm−3)   (5)
A standard dipole antenna has a T-shape and the tip of the coaxial electric wire is connected vertically with a rectilinear radiant antenna with a total length of λ/2. However, the radiant antenna is not necessarily needed to be rectilinear but it may take an oval or U-shape. In either case, resonance occurs at the frequency when the total circumferential length of an antenna is equal to λ/2. In measurement of plasma electron densities, U-shape is preferable than T-shape because the size of the port hole for insertion of the antenna through a vessel wall is small.
FIG. 20 shows a U-shape wire type frequency shift probe as a U-shape antenna, inserted in a plasma 81. FIG. 21 depicts the principle of the U-shape wire type frequency shift probe, described in the aforementioned Non-Patent Document 1. Here, the magnetic force lines generated by the current flowing through a micro-loop(transmitting loop antenna) 89 mounted on the tip of a coaxial cable 88 interlace with the bottom of a U-shape antenna 90 and drive electric current along U-shape wire, from which electromagnetic waves are emitted. The emitted waves are picked up by another micro-loop (receiving loop antenna) 91. Then, I and T are assumed to denote the power incident on the transmitting loop antenna 89 and the transmitting power received on the receiving loop antenna 91, respectively. As shown in FIG. 23(a), when the incident power I is constant independent of frequency f, the transmitting power T becomes resonantly strong at the frequency fr to satisfy the relation, L=λ/4,as shown in equation (2). Here, the width d of the U-shape antenna 90 is designed to be larger than the thickness (several mm) of a sheath generated around the U-shape wire.
The probe in FIG. 21 requires two loops for power transmission and reception as well as two coaxial cables. In contrast with this, the aforementioned Non-Patent Document 2 describes a method to monitor the reflective power R by using one loop and one coaxial cable, as shown in FIG. 22. Here, tip C of core conductor 93 of coaxial cable 92 is connected with point A in the bottom of the U-shape antenna 94 via an arc shape lead wire 95. And, the bottom of U-shape antenna 94 is connected with the surface conductor 96 of the coaxial cable 92 at point G. In this situation, power I incident from the coaxial cable 92 is used to excite the U-shape antenna 94 through the arc shape lead wire. The rest of the power, as reflective power R, is sent back to the power source from the coaxial cable 92. A network analyzer functions to send a micro amount of incident power I to the antenna while sweeping frequencies and monitor reflective power R returning back from the antenna to the power source in the network analyzer. When reflective power R is measured, the power is found to resonantly decreased at the resonant frequency fr, as shown in FIG. 23(c). Plasma electron densities can be determined from this decrease by use of equation (5).
However, in the U-shape antenna 94 acting as a U-shape wire type frequency shift probe, described in Non-Patent Document 2, it is required to connect a lead wire 95 of micro arc shape to the U-shape antenna 94 at the tip of a thin coaxial cable 92. Therefore, it is difficult to fabricate and its mechanical strength is low. Furthermore, the U-shape antenna 94 as a measuring probe has a long thin shape as is the case with a surface wave probe. When this U-shape antenna 94 is protruded into a plasma through the wall of a plasma vessel, it causes a large disturbance in the plasma and it is subject to damage in volume production equipment.
Patent Document 1 describes an example which employs a flat metallic plate several mm wide as a special shape surface wave probe. However, in this case, a simple rectangular metallic plate is adopted just as an antenna of a surface wave probe, which functions on a different principle from that of a frequency shift probe that uses the resonance of electromagnetic waves.